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The most widespread explanation for the name of the mountain is based on the fact that Cadair means "chair" in Welsh. Cadair Idris would thus translate as "the chair of Idris", and Idris is usually taken to be a giant who was said to have used the mountain as an armchair to gaze at the stars. Alternatively, it may refer to Idris ap Gwyddno (or Gweiddno), a 7th-century prince of Meirionnydd who won a battle against the Irish on the mountain.[2][3] Idris ap Gwyddno was in fact referred to as Idris Gawr ("Idris the Giant") in some mediaeval genealogies of Meirionydd.[2] An alternative origin for the name of the mountain, more consistent with the story of Idris ap Gwyddno than that of the mythological giant, is Irishcathair, meaning "city" or "stronghold".[2]

While the name of the mountain is typically spelt Cadair Idris on current maps, it is usually referred to as Cader Idris locally, in both Welsh and English. This is reflected in the name of the local secondary school, Ysgol y Gader (never Ysgol y Gadair).[2] The summit of the mountain is known as Penygader ("top of the chair/stronghold").

This route, which begins in the north from either Dolgellau or the Mawddach estuary, is the easiest but the longest of the main trails. Its length from the mountain's base is 5 kilometres (3.1 mi) with a 600 metres (2,000 ft) climb.

This is the most direct way to the summit as the trail leads straight up the northern face. The 3.8 kilometres (2.4 mi) ascent involves a climb up a 310 metres (1,020 ft) cliff-scree face. However, this part of the Fox's Path has been heavily eroded in recent years making the descent dangerous.

This route starts on the southern side of the mountain near the glacialTal-y-llyn Lake. Hikers using this ascent climb past Llyn Cau and along the rim of Craig Cau (rockwall) to Penygadair. Its length is 4.4 kilometres (2.7 mi) and involves two climbs of over 300 metres (980 ft).

The crater-like shape of Llyn Cau has given rise to the occasional mistaken claim that Cadair Idris is an extinct volcano. This theory was discounted as early as 1872, when Charles Kingsley commented in his book Town Geology :[4]

“

I have been told, for instance, that that wonderful little blue Glas Llyn,[5] under the highest cliff of Snowdon, is the old crater of the mountain; and I have heard people insist that a similar lake, of almost equal grandeur, in the south side of Cader Idris, is a crater likewise. But the fact is not so.

”

The natural bowl-shaped depression was formed by a cirque glacier during the last ice age when snow and ice accumulated in the corries due to avalanches on higher slopes. In these depressions, snow persisted through summer months, and becomes glacial ice. The cirque was up to a square kilometre in size surrounded on three sides by steep cliffs. The highest cliff was the headwall. The fourth side was the "lip" from which the glacier flowed away from the cirque. Over thousands of years ice flowed out through the bowl's opening carving the chair of Caldair Idris.

As the glacier eroded the lip down to the bedrock, there are several tear-drop shaped hills above the edge of Lyn Cau. These geologically important rocky outcrops are called roche moutonnée (English: sheep-like rock) and were formed from the abrasive action of the ice.

The two highest lakes (Llyn Cau and Llyn y Gader) have been dived using SCUBA equipment to explore them and ascertain their depths. Although not extensively studied Llyn y Gader appears to have a maximum depth of around 13 meters. The expeditions to dive these lakes took place in May 2014 and was led by Monty Halls.

Looking at Penygadair (right) from the Pony Path in January 2005. The steep scree route of the Fox's Path is highlighted in sunlight (centre).

There are numerous legends about Cadair Idris. Some nearby lakes are supposed to be bottomless, and anyone who sleeps on its slopes alone will supposedly awaken either a madman or a poet. This tradition (of sleeping on the summit of the Mountain) apparently stems from bardic traditions, where bards would sleep on the mountain in hope of inspiration.[7]

Although the mountain's name is typically taken to refer to the mythological giant Idris, who was said to have been skilled in poetry, astronomy and philosophy,[7] it has sometimes been mistranslated as Arthur's Seat, in reference to King Arthur (and to the hill of the same name in Edinburgh),[8] an idea used by author Susan Cooper in her book The Grey King. However, this translation is mistaken and there is no etymological or traditional connection between Idris and Arthur.

In Welsh mythology, Cadair Idris is also said to be one of the hunting grounds of Gwyn ap Nudd and his Cŵn Annwn. The howling of these huge dogs foretold death to anyone who heard them, the pack sweeping up that person's soul and herding it into the underworld.

The mountain is mentioned several times and is used as a backdrop to the story of Mary Jones and her Bible. This story is an account of a girl from Llanfihangel-y-Pennant at the foot of the mountain who walked 25 miles to Bala in order to buy a bible, which were scarce at the time. Jones' determined journey to get hold of a copy of the book in 1800 was a major factor behind the foundation of the British and Foreign Bible Society in 1804.[9]